Assessing the pKa -Dependent Activity of Hydroxyl Hydrogen Bond Donors in the Organocatalyzed Cycloaddition of Carbon Dioxide to Epoxides: Experimental and Theoretical Study

Catalytic Strategies for the Cycloaddition of CO2 to Epoxides in Aqueous Media to Enhance the Activity and Recyclability of Molecular Organocatalysts

The cycloaddition of CO2 to epoxides to afford versatile and useful cyclic carbonate compounds is a highly investigated method for the nonreductive upcycling of CO2. One of the main focuses of the current research in this area is the discovery of readily available, sustainable, and inexpensive catalysts, and of catalytic methodologies that allow their seamless solvent-free recycling. Water, often regarded as an undesirable pollutant in the cycloaddition process, is progressively emerging as a helpful reaction component. On the one hand, it serves as an inexpensive hydrogen bond donor (HBD) to enhance the performance of ionic compounds; on the other hand, aqueous media allow the development of diverse catalytic protocols that can boost catalytic performance or ease the recycling of molecular catalysts. An overview of the advances in the use of aqueous and biphasic aqueous systems for the cycloaddition of CO2 to epoxides is provided in this work along with recommendations for possible future developments.

Salophen-type Organocatalysts for the Cycloaddition of CO2 and Epoxides under Solvent, Halide, and Metal-Free Conditions

8-Formyl-7-hydroxycoumarin (A) and their derived salophen-type organocatalysts L1, L2, and L3 were used for the synthesis of cyclic carbonates from carbon dioxide (CO2) and epoxides under solvent-, halide-, and metal-free conditions. According to previous optimization tests, L1 and L2 had the best catalytic activity presenting 89 and 92% conversion toward the synthesis of 3-chloropropylene carbonate (2c) using 8 bar CO2, 100 °C at 9 h. Therefore, they were used as organocatalysts to complete the catalytic screening with 11 terminal epoxides (1a-k) exhibiting the highest TOF values of 20 and 22 h-1 using 1c and 1b, respectively. Similarly, they were tested with an internal epoxide, such as cyclohexene oxide (1l) exhibiting 72% conversion, becoming the first salophen organocatalyst to obtain cis-cyclohexane carbonate (2l) in the absence of a cocatalyst. In addition, a reaction mechanism was proposed for the formation of cyclic carbonates based on experimental data and computational techniques; these contributed in establishing a probable role of CO2 pressure along the catalysis and the hydrogen bonds that favor the stabilization of the different intermediates of the reaction.

Organocatalytic Polymers from Affordable and Readily Available Building Blocks for the Cycloaddition of CO2 to Epoxides

The catalytic cycloaddition of CO₂ to epoxides to afford cyclic carbonates as useful monomers, intermediates, solvents, and additives is a continuously growing field of investigation as a way to carry out the atom-economic conversion of CO₂ to value-added products. Metal-free organocatalytic compounds are attractive systems among various catalysts for such transformations because they are inexpensive, nontoxic, and readily available. Herein, we highlight and discuss key advances in the development of polymer-based organocatalytic materials that match these requirements of affordability and availability by considering their synthetic routes, the monomers, and the supports employed. The discussion is organized according to the number (monofunctional versus bifunctional materials) and type of catalytically active moieties, including both halide-based and halide-free systems. Two general synthetic approaches are identified based on the postsynthetic functionalization of polymeric supports or the copolymerization of monomers bearing catalytically active moieties. After a review of the material syntheses and catalytic activities, the chemical and structural features affecting catalytic performance are discussed. Based on such analysis, some strategies for the future design of affordable and readily available polymer-based organocatalysts with enhanced catalytic activity under mild conditions are considered.

Deep Eutectic Solvents as Catalysts for Cyclic Carbonates Synthesis from CO2 and Epoxides

In recent years, the chemical industry has put emphasis on designing or modifying chemical processes that would increasingly meet the requirements of the adopted proecological sustainable development strategy and the principles of green chemistry. The development of cyclic carbonate synthesis from CO₂ and epoxides undoubtedly follows this trend. First, it represents a significant improvement over the older glycol phosgenation method. Second, it uses renewable and naturally abundant carbon dioxide as a raw material. Third, the process is most often solvent-free. However, due to the low reactivity of carbon dioxide, the process of synthesising cyclic carbonates requires the use of a catalyst. The efforts of researchers are mainly focused on the search for new, effective catalysts that will enable this reaction to be carried out under mild conditions with high efficiency and selectivity. Recently, deep eutectic solvents (DES) have become the subject of interest as potential effective, cheap, and biodegradable catalysts for this process. The work presents an up-to-date overview of the method of cyclic carbonate synthesis from CO₂ and epoxides with the use of DES as catalysts.

Theoretical Perspectives in Organocatalysis

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

Simple Halogen-Free, Biobased Organic Salts Convert Glycidol to Glycerol Carbonate under Atmospheric CO2 Pressure

Glycerol carbonate (GC) has emerged as an attractive synthetic target due to various promising technological applications. Among several viable strategies to produce GC from CO₂ and glycerol and its derivatives, the cycloaddition of CO₂ to glycidol represents an atom-economic an efficient strategy that can proceed via a halide-free manifold through a proton-shuttling mechanism. Here, it was shown that the synthesis of GC can be promoted by bio-based and readily available organic salts leading to quantitative GC formation under atmospheric CO₂ pressure and moderate temperatures. Comparative and mechanistic experiments using sodium citrate as the most efficient catalyst highlighted the role of both hydrogen bond donor and weakly basic sites in the organic salt towards GC formation. The citrate salt was also used as a catalyst for the conversion of other epoxy alcohols. Importantly, the discovery that homogeneous organic salts catalyze the target reaction inspired us to use metal alginates as heterogeneous and recoverable bio-based catalysts for the same process.

Deprotonation from an OH on myo-Inositol Promoted by μ2-Bridges with Possible Regioselectivity/Chiral Selectivity

Single-crystal structures of myo-inositol complexes with erbium ([Er₂(C₆H₁₁O₆)₂(H₂O)₅Cl₂]Cl₂(H₂O)₄, denoted ErI hereafter) and strontium (Sr(C₆H₁₂O₆)₂(H₂O)₂Cl₂, denoted SrI hereafter) are described. In ErI, deprotonation occurs on an OH of myo-inositol, although the complex is synthesized in an acidic solution, and the pK_a values of all of the OHs in myo-inositol are larger than 12. The deprotonated OH is involved in a μ₂-bridge. The polarization from two Er³⁺ ions activates the chemically relatively inert OH and promotes deprotonation. In the stable conformation of myo-inositol, there are five equatorial OHs and one axial OH. The deprotonation occurs on the only axial OH, suggesting that the deprotonation possesses characteristics of regioselectivity/chiral selectivity. Two Er³⁺ ions in the μ₂-bridge are stabilized by five-membered rings formed by chelating Er³⁺ with an O-C-C-O moiety. As revealed by the X-ray crystallography study, the absolute values of the O-C-C-O torsion angles decrease from ∼60 to ∼45° upon chelating. Since the O-C-C-O moiety is within a six-membered ring, the variation of the torsion angle may exert distortion of the chair conformation. Quantum chemistry calculation results indicate that an axial OH flanked by two equatorial OHs (double ax-eq motif) is favorable for the formation of a μ₂-bridge, accounting for the selectivity. The double ax-eq motif may be used in a rational design of high-performance catalysts where deprotonation with high regioselectivity/chiral selectivity is carried out.

CO2 fixation into cyclic carbonates catalyzed by single-site aprotic organocatalysts

The catalytic activity of a series of onium salts for the synthesis of cyclic carbonates have been investigated experimentally and theoretically.

Unexpected Deprotonation from a Chemically Inert OH Group Promoted by Metal Ions in Lanthanide-Erythritol Complexes

Single-crystal structures of five lanthanide-erythritol complexes are reported. The analysis of the chemical compositions and scrutinization of structural features in the single-crystal data of the complexes led us to find that unexpected deprotonation occurs on the OH group of erythritol of three complexes. Considering these complexes were prepared in acidic environments, where spontaneous ionization on an OH group is suppressed, we suggest metal ions play an important role in promoting the proton transfer. To find out why the chemically inert OH is activated, the single-crystal structures of 63 rare-earth complexes containing organic ligands with multiple hydroxyl groups (OLMHs) were surveyed. The formation of μ₂-bridges turns out to be directly relevant to the occurrence of deprotonation. When an OH group from an OLMH molecule participates in the formation of a μ₂-bridge, the polarization ability of the metal ions becomes strong enough to promote the deprotonation on the OH group. The above structural characteristics may be useful in the rational design of catalysts that can activate the chemically inert OH group and promote the relevant chemical conversions.

Heterobimetallic rare earth metal-zinc catalysts for reactions of epoxides and CO2 under ambient conditions

Four homodinuclear rare earth metal (RE) complexes 1-4 bearing a multidentate diglycolamine-bridged bis(phenolate) ligand were synthesized. In addition, seven heterobimetallic RE-Zn complexes 5-11 were prepared through a one-pot strategy. In these heterobimetallic complexes, two RE centers are bridged by either Zn(OAc)₂ or Zn(OBn)₂ moieties. All complexes were characterized by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy (in the case of diamagnetic complexes 1, 4, 7 and 11). Moreover, the multi-nuclear structures of complexes 4 and 11 in solution were also studied by ¹H DOSY spectroscopy. These complexes were applied in catalyzing the coupling reaction of carbon dioxide (CO₂) with epoxides. Zn(OAc)₂- and Zn(OBn)₂-bridged heterobimetallic complexes showed comparable catalytic activities under ambient conditions and were more active than monometallic RE complexes. Significant synergistic effect in heterobimetallic complexes is observed. Mono-substituted epoxides were converted into cyclic carbonates under 1 atm CO₂ at 25 °C in 88-96% yields, whereas di-substituted epoxides reacted under 1 atm CO₂ at higher temperatures in 40-80% yields.

Cycloaddition mechanisms of CO2 and epoxide catalyzed by salophen – an organocatalyst free from metals and halides

The coupling mechanism of CO₂ and epichlorohydrin catalyzed by salophen is computed. A neutral concerted bifunctional mechanism of phenolate as nucleophile and phenol as H-bonding donor in epoxide ring-opening and CO₂ addition is suggested.

An octahedral cobalt(iii) complex based on cheap 1,2-phenylenediamine as a bifunctional metal-templated hydrogen bond donor catalyst for fixation of CO2 with epoxides under ambient conditions

An octahedral cobalt(iii) complex based on cheap 1,2-phenylenediamine operates as an efficient bifunctional hydrogen bond donor catalyst in cycloaddition of epoxides with CO₂ under ambient conditions and solvent- and co-catalyst-free conditions.

Cycloaddition of CO2 to epoxides by highly nucleophilic 4-aminopyridines: establishing a relationship between carbon basicity and catalytic performance by experimental and DFT investigations

New highly nucleophilic homogeneous and heterogeneous catalysts based on the 3,4-diaminopyridine scaffold are reported for the halogen-free cycloaddition of CO₂ to epoxides.

Rapid conversion of CO2 and propylene oxide into propylene carbonate over acetic acid/KI under relatively mild conditions

The catalytic system of acetic acid/KI was studied to demonstrate high activity for completely converting propylene oxide into propylene carbonate within a quite short time of 15 min under the relatively mild conditions of 0.9 MPa CO2 and 90 °C.

Aliphatic carboxylic acid as a hydrogen-bond donor for converting CO2 and epoxide into cyclic carbonate under mild conditions

The catalytic systems of aliphatic carboxylic acids/quaternary ammonium halides could efficiently convert the coupling of CO₂ and epoxide into cyclic carbonates under mild conditions (80 °C and 4 bar CO₂).

Co(III)-Salen immobilized cellulose nanocrystals for efficient catalytic CO2 fixation into cyclic carbonates under mild conditions

Searching for green, recyclable and highly efficient catalyst for the synthesis of cyclic carbonates from CO₂ is of great importance because it is profitable for reducing the greenhouse effects and meets the principles of green chemistry. Herein, a series of cellulose nanocrystals, either the pristine or modified ones (TEMPO oxidized and Co(III)salen immobilized), were explored as catalysts for cycloaddition of epoxides and carbon dioxide. The impact of surface properties on the performance of the as-made catalysts was investigated. Co(III)-salen grafted cellulose nanocrystals was proven to be the most effective catalyst in this study, which could afford excellent yield up to 99 % after 24 h even under low CO₂ pressures of 0.1 MPa. They can be easily recovered and reused for at least 4 times, demonstrating their excellent stability. We found that the surface functional groups such as enriched sulfate or carboxylic groups could also account for the enhanced catalytic activity. This work highlights the applications of green and sustainable nanoparticles in a cycloaddition reaction and offers a sustainable solution in industrial catalysis related to CO₂ conversions.

Triazine-based Organic Polymer-catalysed Conversion of Epoxide to Cyclic Carbonate under Ambient CO2 Pressure

In this work we have achieved epoxide to cyclic carbonate conversion using a metal-free polymeric catalyst under ambient CO₂ pressure (1.02 atm) using a balloon setup. The triazine containing polymer (CYA-ANIS) was prepared from cyanuric chloride (CYA-Cl) and o-dianisidine (ANIS) in anhydrous DMF as solvent by refluxing under the N₂ gas environment. The presence of triazine and amine functional groups in the polymer results in the adsorption of CO₂ up to 7 cc/g at 273 K. This inspired us to utilize the polymer for the conversion of a series of functionalised epoxides into their corresponding cyclic carbonates in the presence of tetrabutyl ammonium iodide (TBAI) as co-catalyst. The product has wide range of applications like solvent in lithium ion battery, precursor for polycarbonate, etc. The catalyst was efficient for the conversion of different mono and di-epoxides into their corresponding cyclic carbonates under atmospheric pressure in the presence of TBAI as co-catalyst. The study indicates that epoxide attached with electron withdrawing groups (like, CH₂ Cl, glycidyl ether, etc.) displayed better conversion compared to simple alkane chain attached epoxides. This is mainly due to the stabilization of electron rich intermediates produced during the reaction (e. g. epoxide ring opening or CO₂ incorporation into the halo-alkoxide anion). This catalyst mixture was capable to maintain its reactivity up to five cycles without losing its activity. Post catalytic characterization clearly supports the heterogeneous and recyclable nature of the catalyst.

Intertwined chemistry of hydroxyl hydrogen-bond donors, epoxides and isocyanates in the organocatalytic synthesis of oxazolidinones versus isocyanurates: rational catalytic investigation and mechanistic understanding

The efficiency and chemoselectivity of the cycloaddition of isocyanates to epoxides to afford oxazolidinones were investigated using hydroxyl hydrogen-bond donors as organocatalysts.

Facile synthesis of crystalline viologen-based porous ionic polymers with hydrogen-bonded water for efficient catalytic CO2 fixation under ambient conditions

In this work, we report a series of crystalline viologen-based porous ionic polymers (denoted VIP-X, X = Cl or Br), that have in situ formed dicationic viologens paired with halogen anions and intrinsic hydrogen-bonded water molecules, towards metal-free heterogeneous catalytic conversion of carbon dioxide (CO₂) under mild conditions. The targeted VIP-X materials were facilely constructed via the Menshutkin reaction of 4,4′-bipyridine with 4,4′-bis(bromomethyl)biphenyl (BCBMP) or 4,4′-bis(chloromethyl)biphenyl (BBMBP) monomers. Their crystalline and porous structures, morphological features and chemical structures and compositions were fully characterized by various advanced techniques. The optimal catalyst VIP-Br afforded a high yield of 99% in the synthesis of cyclic carbonate by CO₂ cycloaddition with epichlorohydrin under atmospheric pressure (1 bar) and a low temperature (40 °C), while other various epoxides could be also converted into cyclic carbonates under mild conditions. Moreover, the catalyst VIP-Br could be separated easily and reused with good stability. The remarkable catalytic performance could be attributed to the synergistic effect of the enriched Br⁻ anions and available hydrogen bond donors –OH groups coming from H-bonded water molecules.

Viologen-based porous ionic polymers with halogen anions and hydrogen-bonded water were constructed for efficient catalytic CO₂ fixation under mild conditions.

Molecular design and experimental study on synergistic catalysts for the synthesis of cyclocarbonate from styrene oxide and CO2

Taking the reaction between styrene oxide and CO₂ to yield cyclocarbonate as the target, the activities of synergistic catalysts, which are composed of Br⁻ and alcohol compounds serving as hydrogen bond donors (HBDs), were predicted by DFT calculations and confirmed by subsequent experiments.

Evaluation and understanding the performances of various derivatives of carbonyl-stabilized phosphonium ylides in CO2 transformation to cyclic carbonates

The kinetic and mechanism evaluations of the formation of cyclic carbonates by carbonyl-stabilized phosphonium ylides as an efficient and new class of organocatalysts are the main purposes of this research.

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO₂ activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

Mechanistic Studies of CO2 Cycloaddition Reaction Catalyzed by Amine-Functionalized Ionic Liquids

The homogeneous cycloaddition reaction of CO₂ and epichlorohydrin catalyzed by amine-functionalized ionic liquid (AFIL) to yield cyclic carbonate is reported in this study. The AFIL has the dual function of ionic liquid and organic base. The experimental study indicates that AFIL can efficiently catalyze the conversion of CO₂ and epichlorohydrin to the product 3-chloro-1,2-propylene. The mechanistic study based on DFT calculations reveals that the imidazolium ring in AFIL primarily catalyzes the ring-opening reaction of epichlorohydrin, while the protonated amine group is responsible for stabilizing the Br⁻ anion in the nucleophilic attack. This study provides a deep insight into the catalytic roles of AFIL and also inspires us to design efficient dual function catalysts for CO₂ utilization.

The Role of Water Revisited and Enhanced: A Sustainable Catalytic System for the Conversion of CO2 into Cyclic Carbonates under Mild Conditions

The role of water as highly effective hydrogen-bond donor (HBD) for promoting the coupling reaction of CO₂ with a variety of epoxides was demonstrated under very mild conditions (25-60 °C, 2-10 bar CO₂ ). Water led to a dramatic increase in the cyclic carbonate yield when employed in combination with tetrabutylammonium iodide (Bu₄ NI) whereas it had a detrimental effect with the corresponding bromide and chloride salts. The efficiency of water in promoting the activity of the organic halide was compared with three state-of-the-art hydrogen bond donors, that is, phenol, gallic acid and ascorbic acid. Although water required higher molar loadings compared to these organic hydrogen-bond donors to achieve a similar degree of conversion of CO₂ and styrene oxide into the corresponding cyclic carbonate under the same, mild reaction conditions, its environmental friendliness and much lower cost make it a very attractive alternative as hydrogen-bond donor. The effect of different parameters such as the amount of water, CO₂ pressure, reaction temperature, and nature of the organic halide used as catalyst was investigated by using a high-throughput reactor unit. The highest catalytic activity was achieved with either Bu₄ NI or bis(triphenylphosphine)iminium iodide (PPNI): with both systems, the cyclic carbonate yield at 45 °C with different epoxide substrates could be increased by a factor of two or more by adding water as a promoter, retaining high selectivity. Water was an effective hydrogen-bond donor even at room temperature, allowing to reach 85 % conversion of propylene oxide with full selectivity towards propylene carbonate in combination with Bu₄ NI (3 mol %). For the conversion of epoxides in which PPNI is poorly soluble, the addition of a cyclic carbonate as solvent allowed the formation of a homogeneous solution, leading to enhanced product yield.

Recent Advances in the Chemical Fixation of Carbon Dioxide: A Green Route to Carbonylated Heterocycle Synthesis

Carbon dioxide produced by human activities is one of the main contributions responsible for the greenhouse effect, which is modifying the Earth’s climate. Therefore, post-combustion CO2 capture and its conversion into high value-added chemicals are integral parts of today’s green industry. On the other hand, carbon dioxide is a ubiquitous, cheap, abundant, non-toxic, non-flammable and renewable C1 source. Among CO2 usages, this review aims to summarize and discuss the advances in the reaction of CO2, in the synthesis of cyclic carbonates, carbamates, and ureas appeared in the literature since 2017.

Non-Oxidative Enzymatic (De)Carboxylation of (Hetero)Aromatics and Acrylic Acid Derivatives

The utilization of carbon dioxide as a C1-building block for the production of valuable chemicals has recently attracted much interest. Whereas chemical CO2 fixation is dominated by C-O and C-N bond forming reactions, the development of novel concepts for the carboxylation of C-nucleophiles, which leads to the formation of carboxylic acids, is highly desired. Beside transition metal catalysis, biocatalysis has emerged as an attractive method for the highly regioselective (de)carboxylation of electron-rich (hetero)aromatics, which has been recently further expanded to include conjugated α,β-unsaturated (acrylic) acid derivatives. Depending on the type of substrate, different classes of enzymes have been explored for (i) the ortho-carboxylation of phenols catalyzed by metal-dependent ortho-benzoic acid decarboxylases and (ii) the side-chain carboxylation of para-hydroxystyrenes mediated by metal-independent phenolic acid decarboxylases. Just recently, the portfolio of bio-carboxylation reactions was complemented by (iii) the para-carboxylation of phenols and the decarboxylation of electron-rich heterocyclic and acrylic acid derivatives mediated by prenylated FMN-dependent decarboxylases, which is the main focus of this review. Bio(de)carboxylation processes proceed under physiological reaction conditions employing bicarbonate or (pressurized) CO2 when running in the energetically uphill carboxylation direction. Aiming to facilitate the application of these enzymes in preparative-scale biotransformations, their catalytic mechanism and substrate scope are analyzed in this review.

Chiral Bifunctional Metalloporphyrin Catalysts for Kinetic Resolution of Epoxides with Carbon Dioxide

Chiral binaphthyl-strapped Zn(II) porphyrins with triazolium halide units were synthesized as bifunctional catalysts for kinetic resolution of epoxides with CO₂. Several catalysts were screened by changing the linker length and nucleophilic counteranions, and the optimized catalyst accelerated the enantioselective reaction at ambient temperature to produce optically active cyclic carbonates and epoxides.